1. Field of the Invention
The present invention relates to the field of displaying technology, and in particular to a high transmittance PSVA (Polymer Stabilized Vertical Alignment) liquid crystal display panel and a manufacturing method thereof.
2. The Related Arts
Active thin-film transistor liquid crystal displays (TFT-LCDs) have been recently developed swiftly and have had wide applications recently. Mainstream TFT-LCDs currently available in the market belong to three categories, which are respectively twisted nematic (TN) or super twisted nematic (STN) type, in-plane switching (IPS) type, and vertical alignment (VA) type. Among them, VA type liquid crystal displays have extremely high contrast as compared to other types and may generally reach 4000-8000. This provides extremely wide applications in large-sized displays, such as televisions.
The reason that the VA type liquid crystal displays have the extremely high contrast is because liquid crystal molecules are arranged perpendicular to a surface of a substrate and have no phase difference in a dark state where no electricity is applied so as to have extremely low leakage of light and very low dark state brightness. According to contrast calculation equation, the lower the dark state brightness is, the higher the contrast will be. To allow the liquid crystal molecules in a VA type liquid crystal display panel to be arranged perpendicular to the surface of the substrate, the liquid crystal molecules must be subjected to alignment, of which the most common way is to coat a vertical alignment agent (which is a polymer material of polyimide (PI), a PI solution) on specific areas of upper and lower substrates and then the substrates are baked for a long time in a predetermined temperature to dry a solvent of the alignment agent thereby forming a PI alignment layer on the surfaces of the substrates. As shown in FIG. 1, a conventional VA type liquid crystal display panel comprises: an upper substrate 10, a lower substrate 20 opposite to the upper substrate 10, a liquid crystal layer 40 interposed between the lower substrate 10 and the upper substrate 20, and a PI alignment layer 30 formed on a surface of the upper substrate 10 that faces the lower substrate 20 and a surface of the lower substrate 20 that faces the upper substrate 10. Since the VA type liquid crystal display panel involves liquid crystal that are vertically rotating, birefringence of the liquid crystal molecules is relatively large, leading to a severer issue of color shift in a large view angle.
To provide a VA type liquid crystal display panel with bettered characteristics in wide view angles and to improve the issue of color shift, multi-domain VA (MVA) technology is often adopted, in which a sub-pixel is divided into multiple zones and the liquid crystal of each zone is caused to tilt in different direction when a voltage is applied so that the result of viewing from different angles would approach homogeneity and consistency. Various ways may be used to provide the MVA technology. As shown in FIGS. 2 and 3, one of such ways is to make an ITO pixel electrode 70 at one side in the form of a star-shaped pattern, while a common electrode 80 is made a planar electrode that has a uniform thickness and is continuous without interruption. Due to the unique ITO pixel electrode pattern, an inclined electrical field may be generated to induce the liquid crystal molecules 40 in different zones to tilt in different directions.
FIG. 2 is a top plan view of one side of a lower substrate 20 of a MVA type liquid crystal display panel, in which reference numerals 210 and 220 are respectively a scan line and a data line. A sub-pixel is divided into four zones. In each zone, the ITO pixel electrode 70 is formed of a pattern comprising pixel electrode branches extending in a different direction and alternating spacing slits. FIG. 3 is a cross-sectional view of the MVA type liquid crystal display panel taken along line A-A of FIG. 2, wherein the pixel electrode 70 that comprises the slits is formed on a planar lower passivation layer 60 and the PI alignment layer 30 that covers the pixel electrode 70 has an irregular surface, while a common electrical 80 that is in a planar form is provided on a planar upper passivation layer 90 and the PI alignment layer 30 that covers the common electrode 80 has a surface that shows a straight planar surface.
With the progress of technology, a kind of MVA type liquid crystal display panel that requires no PI alignment layer is available and is referred to as a polymer stabilized vertical alignment (PSVA) liquid crystal display panel. As shown in FIGS. 4-6, a pixel electrode 400 that is processed to form a star-shaped pattern is arranged on a lower substrate 200, while a planar common electrode 300 is arranged on an upper substrate 100. Polymer projections 500 attached to surfaces of the pixel electrode 400 and the common electrode 300 provide liquid crystal molecules 700 contained in a liquid crystal layer with pre-tilt angles in given directions. FIGS. 7-9 are schematic views illustrating key manufacturing processes of the conventional PSVA liquid crystal display panel. Firstly, a planar common electrode 300 is formed on an upper substrate 100 and a pixel electrode 400 that is processed to form a star-shaped pattern is manufactured on a lower substrate 200. The upper and lower substrates are laminated together and a mixture of liquid crystal molecules 700 and polymerizable monomers 500′ is filled therein. A voltage is then applied to the common electrode 300 and the pixel electrode 400 to cause the liquid crystal molecules 700 to tilt in given directions, namely tilting in the directions of the slits of the pixel electrode 400 that is processed to form a star-shaped pattern. Then, ultraviolet (UV) ray is irradiated to cause reaction of the polymerizable monomers 500′ to form polymer projections 500 attached to surfaces of the pixel electrode 400 and the common electrode 300 thereby providing the liquid crystal molecules 700 with pre-tilt angles in given directions. In the above manufacturing process, the pixel electrode 400 that is processed to form a star-shaped pattern has an important effect. If the pixel electrode does not comprise the star-shaped pattern, then upon the application of voltage, the tilting directions of the liquid crystal molecules would be random and uncontrollable.
Since the pixel electrode 400 that is processed to form a star-shaped pattern comprise a pattern that comprises pixel electrode branches that extend in various directions and alternate with the slits, the electric field formed thereby with respect to the opposite common electrode 300 is not uniform, the electric field corresponding to an area of the pixel electrode branch is apparently stronger than the electric field corresponding to an area of the slit thereby leading to a phenomenon of non-uniform brightness occurring in the pixel.
Further, according to the transmittance equation of the VA type liquid crystal display panel:T=(½)sin22Δφ sin2(Γ/2)  (1)where T is transmittance; Δφ is an included angle between a long axis of liquid crystal and a polarizer, which provides the maximum efficiency at 45°; Γ is phase difference, which is the effect of modulation of the rotation of liquid crystal molecules with respect to polarization light under an electric field.
The formula for Γ is as follows:Γ=cos(a)*2π*Δn*d/λ  (2)where a is an included angle between the long axis of liquid crystal molecule and a normal of a substrate, which is determined by the electric field applied to the liquid crystal molecules; d is thickness of a liquid crystal cell; and Δn is refractive index difference between long and short axes of the liquid crystal.
It can be known from equation (1) that when Γ is π, the transmittance is maximum. Combining equation (1) with equation (2) provides that with the liquid crystal cell thickness d being fixed, the transmittance is determined by the electric field applied to the liquid crystal. When Γ is smaller than π, the greater the electric field is, the greater the transmittance would be; and when Γ is greater than π, the greater the electric field is, the smaller the transmittance would be.
Since the electric fields associated with the areas corresponding to the pixel electrode branches and the slits are not consistent, it is generally not possible to make the transmittance of these two areas simultaneously reach the maximum value. In other words, when Γ of the pixel electrode branch is π, the value of Γ of the area corresponding to the slit is smaller than π; and when the value of Γ of the area corresponding to the slit reaches π, the value of Γ of the area associated with the pixel electrode branch has already exceeded π. Thus, the entire pixel area cannot simultaneously reach the maximum transmittance and thus, it is not possible to achieve the maximum transmittance of a liquid crystal display panel and the brightness of the liquid crystal display panel is non-uniform.